Radio-isotopic radiological sources produce relatively constant fluxes of desired radiation (i.e. ionizing radiation) such as alpha, beta, gamma rays, and even neutron emission, through radioactive decay of specific radioactive isotopes in the source. These radio-isotopes are typically created from the irradiation of stable elements in nuclear reactors or accelerators beamlines, or are by-products of fission reactions in reactors. By mating such radio-isotopes with targets, radio-isotopic neutron sources can be created. For example, AmBe neutron sources consist of radioactive americium-241 (241Am) isotope material mixed with a beryllium-9 (9Be) target material. 241Am produces ˜5 MeV alpha particles as part of its decay process, or He2+ ions, which impinge on the beryllium nuclei and produces isotropic ˜1 to 5 MeV neutrons through the 9Be(α, n)12C nuclear reaction. AmBe sources typically produce 106 n/s or greater using ˜Ci level or higher of 241Am, and are commonly used in the well logging industry for characterizing and assessing (via neutron activation analysis) the geology surrounding exploratory boreholes in oil drilling and mining operations. Other alpha emitters, such as 210Po have also been used with beryllium targets for neutron production. And some other examples of radio-isotopic radiological sources include 137Cs, 60Co, 192Ir (in the ˜kCi or higher range) which are MeV level gamma ray emitters (through radioactive decay) which can be use for example for radiotherapy and cancer treatment, or for sterilization of foodstuffs and equipment. Generally, while such radio-isotopic radiological sources have the advantage of a long useful life, they cannot be turned off, and as such require the source be contained in bulky shielding. For example 241Am has a half-life of 458 years, and Californium-252 (252Cf), another typical neutron source, has a half-life of about 2.6 years. In addition, radio-isotopic neutron sources in general cannot be pulsed, and the energy spectrum of the emitted neutrons is broad and peaks at energies below the threshold for some important reactions. Radio-isotopic radiological sources also raise concerns regarding the potential dispersal of spent-and-discarded or stolen radio-isotopic neutron sources by terrorist activity.
One common type of non-radio-isotopic neutron source solution, known as “neutron generators,” are small neutron source devices which fuse isotopes of hydrogen together using a compact linear accelerator. The fusion reactions take place in these devices by accelerating deuterium, tritium, or mixture thereof into a metal hydride target which also contains deuterium, tritium or mixture. Fusion of deuterium atoms (D-D reaction) results in the formation of a He-3 (3He) ion and a neutron with a kinetic energy of approximately 2.5 MeV. Fusion of a deuterium and a tritium atom (D-T reaction) results in the formation of a He-4 (4He) ion and a neutron with a kinetic energy of approximately 14.1 MeV. Typically, a low gas pressure cold cathode ion source utilizing crossed electric and magnetic fields, known as a Penning ion source, is used to generate the deuterium, tritium, or mixture in these LINAC-based neutron generators. Ions generated by the Penning ion source are accelerated by the potential difference between an exit cathode and an accelerating electrode into the tritium or deuterium target to emit neutrons. The target is typically a thin film of a metal such as titanium, scandium, or zirconium which, when combined with hydrogen or its isotopes, forms stable metal hydrides having two hydrogen (D or T) atoms per metal atom and allows the target to have extremely high densities of hydrogen. While this approach is known to produce high levels of neutrons (e.g. 109 n/s), the neutron energy spectrum is much harder and substantially different than the AmBe spectrum (˜1-5 MeV range), and therefore produces different responses from materials when compared to interrogation using AmBe neutrons, which would effectively render inapplicable the extensive industry knowledge base of subsurface strata gained from AmBe sources.
Another known type of non-radio-isotopic neutron source solution is also based on the D-D or D-T fusion reaction, but uses a dense plasma focus (DPF) in a deuterium or deuterium-tritium gas medium to faun D-D or D-T fusion neutrons by electromagnetic acceleration and compression of intense z-pinch plasma, i.e. a column of hot, compressed plasma formed at an anode tip and along a z-axis of the DPF. FIG. 1 illustrates a cross-section view of a typical DPF-based D-D neutron source known in the art, generally indicated at reference character 10, and having a typical Mather-type DPF configuration comprised of an open-ended coaxial head or gun having a center/inner anode 11 and a coaxial outer cathode 12 separated by a coaxial insulator 13. DPF dimensions may be, for example, approximately 1-10 cm long and 1-10 cm diameter. A deuterium fill gas at the Ton level is normally used. And a moderate to high-power pulse foaming network (PFN) made of fast capacitors with energy ranges in the ˜100 J to 1 Mj range, generally represented by the simple circuit 14, drives the plasma and the pinch effect at the anode tip.
During DPF operation, plasma current sheets in a coaxial configuration are first formed from flashover breakdown along insulator surfaces on the center/inner anode 11 and accelerated forward towards the open end through J×B forces. At the open end of the coaxial gun, the sheets expand and collide to form a ˜cm long high density (ne˜1019-1020/cc, Te˜1-10 keV) z-pinch plasma on the tip of the center/inner anode. In this region, the z-pinch is typically confined for ˜100 ns and intense x-rays, ion, and electron beams are generated through a complex combination of non-linear instabilities and other mechanisms that result in effective acceleration gradients of ˜100 MV/m. In particular, ion and electron beams are axially emitted in opposite directions during the pinch.
FIG. 1 illustrates the sequence of plasma sheath dynamics, labeled from (I) to (IV) leading to z-pinch formation and D-D neutron formation and acceleration. In particular, an initial plasma sheath (I) forms from a flashover of the coaxial insulator 13, then plasma sheaths (II) and (III) accelerate forward along the central anode toward the anode tip during the rundown phase, and when it reaches the tip end of the anode the sheath axisymetrically collapses at (IV) during the runover phase to form the centimeter-scale fast z-pinch on axis, shown as circle 15, at the tip of the center anode 11. D-D neutron output is shown at arrow 16 originating from the z-pinch, and axially emitted along the z-axis due to the acceleration gradient. D-D neutron outputs up to ˜1012 in a ˜100 ns long pulse have been demonstrated, giving high peak neutron rates of ˜1019 n/s with average rates up to ˜1012 n/s using a 1 Hz repetition rate. However, and similar to neutron generators, DPF based D-D neutrons produce different responses from materials when compared to interrogation using AmBe neutrons, which would again effectively render inapplicable the extensive industry knowledge base of subsurface strata gained from AmBe sources. Moreover, they are not tuned for acceleration, use only D-D or D-T for neutrons, and do not provide multi-staging of z-pinch focusing.
It would be advantageous to provide a non-radio-isotopic radiological source, such as a neutron source, that is based on pulsed DPF acceleration of charged particles to strike an ionizing-radiation producible target, e.g. alpha particles (He2+) into a beryllium target for neutron emission having energy spectra similar to an americium-beryllium (AmBe) neutron source. Moreover, it would be advantageous to provide a DPF-accelerated non-radio-isotopic radiological source capable of multi-staging dense plasma z-pinch focusing to enhance acceleration energies, tuning, and control.